1. Field of the Invention
The present invention relates to a fuel injection pump for an internal combustion engine (hereinafter called engine), in particular, a construction of the pump with no hammer noises and less frictional wear.
2. Description of Related Art
In a conventional fuel injection pump, a driving force is transmitted to a camshaft via a belt or a gear from an engine crankshaft. A cam attached to the camshaft drives a movable member so that the movable member makes a reciprocating movement. Accordingly, the fuel injection pump pressurizes and delivers fuel sucked in a fuel pressure chamber.
When the camshaft is rotating, the camshaft often moves in an axial direction thereof so that the cam is likely to hit a surface of a housing and make hammer noises. To cope with the hammer noises, it is preferable that the fuel injection pump is provided with biasing means for urging the camshaft in one axial direction thereof to prevent the camshaft from moving to and fro in an axial direction.
In a small engine whose driving force is relatively small, a belt has been used to transmit the driving force to the camshaft. In a large engine whose driving force is relatively large, a gear has been generally used to transmit the driving force to the camshaft from the engine crankshaft. In the case where the gear is used for transmitting the driving force, a helical gear is preferable since the helical gear acts as the biasing means for urging the camshaft in the one axial direction thereof. Further, the helical gear serves to minimize a backrush in mesh so that, when the cam drives the movable member, the driving force transmitted to the cam is relatively even.
In this case, while the camshaft is urged in the one axial direction thereof, a stopper surface, which comes in contact with the cam, is provided in the housing for restricting an axial movement of the camshaft. Accordingly, the cam slides the stopper surface according to a rotation of the camshaft. Face pressure at the respective portions where the cam and the stopper surface are in slidable contact with each other differ depending on a radial distance from an axis of the camshaft and, in particular, face pressure at a lower cam rise portion becomes higher. Further, a region of the stopper surface in slidable contact with the cam varies according to the rotation of the camshaft due to hill and dale profile of the cam. Therefore, frictional wear of both of the cam and the stopper surface are likely to be accelerated so that endurances of the cam and the stopper surface become shorter.
Further, even if the camshaft is biased in the one axial direction and the cam is in slidable contact with the stopper surface, the camshaft is sometimes moved to the other axial direction due to reaction. To limit a backward movement of the camshaft, it is preferable that a clearance between the cam and the housing on an opposite side to the stopper surface is as small as possible. However, it is rather difficult to adjust adequately the clearance between the cam and the housing.